1. Field of the Invention
This present invention relates to an electrophotographic image forming apparatus, and more particularly relates to an image forming apparatus capable of forming a high-quality image without image defects even onto a rough surface sheet or a sheet wrinkled by heat, using a transfer unit having a roller-shaped or belt-shaped transfer member.
2. Description of the Related Art
A continuous form printer has often been used for computer output printing on business forms. Recently, the continuous form printer has also been used for various purpose printing of direct mail, invoices, manuals, books, and so on, by taking advantage of the features of a high speed variable information printer. With an increase in various uses, the continuous form printer is required to support various kinds of webs (recording medium) such as thin paper, thick paper, high-quality paper, and rough low quality paper.
Meanwhile, from a standpoint of saving resources, the needs of double-side printing are increasing. Double-side printing is performed by using two printers (tandem printer), a first printer prints on a first face of a web, the web is reversed, and the second printer prints on a second face of the web. In the case of various uses, the transfer performance is relevant to obtaining an image quality without image defects.
However, in tandem printing, a web may be damaged by heat from fixing at the first printer, and the web may become irregular (concavity and convexity) due to thermal shrinkage.
FIG. 9 is a schematic diagram of a tandem printing device capable of duplex printing on a continuous sheet. As shown in FIG. 9, the tandem printer has a first continuous sheet printer P1 forming a first image 70a on a first surface of a web 7 which is a continuous long-sheet, and a second continuous sheet printer P2 forming a second image 70b on a second surface of the web 7.
The continuous sheet printers P1 and P2 feed a recording medium at high speed (0.5-2 m/s) for printing. The recording medium has perforations between pages or is a roll sheet without perforations, and is different from cut sheets.
The first image 70a is formed on a side of the web 7 by the first printer P1 which has an image forming apparatus containing a developing unit 4a including a photoconductor 1a. The web 7 is reversed by a turn bar T. Then the second image 70b is printed on the other side of the web 7 by the second printer P2 which has an image forming apparatus containing a developing unit 4b including a photoconductor 1b. 
Thus, in a case that images are printed on both sides of the web 7, image defects may be created when second images are formed by the second printer P2 on the other side of the web part which is stopped at area B in FIG. 9 where a fixing member fixing the image 70a on the web 7 is placed, for example.
FIG. 10 is a schematic enlarged diagram of the area B in FIG. 9 which shows the web-stop state of the fixing member. As shown in FIG. 9, the fixing member includes a pre-heater 13 preheating the web 7, a heat roller 14 with a heater, and a backup roller 15 pressing the heat roller 14.
Both the pre-heater 13 and the heat roller 14 continue to supply heat to the web 7 during continuous printing. When the printing operation is stopped, the heaters are turned off. But, those heaters heated the web 7, because of natural cooling. Thus, parts of the web 7 on or near the pre-heater 13 shrink, for example, in the direction of the arrow in FIG. 10, and these heated parts create irregularities due to heat. Therefore, after printing is restarted, the parts of the web 7 having been stopped on the pre-heater 13 of the first printer P1 have image defects and are passed through an image forming area of the second printer P2.
In order to solve such a problem, for example, Japanese Unexamined Patent Application Publication No. 2007-47195 describes that a transfer-assist blade presses a web towards a surface of the photoconductor. By using the transfer-assist blade, a gap between irregularities of the web surface and a photoconductor can be decreased, and a good result for transferring may be obtained. However, in the case of double-side printing, toner images have already been formed on a first surface of a web, when toner images are printed on a second surface of the web. When the web is fed to a transfer member in a second printer, the surface contacting a transfer-assist blade is the first surface on which toner images have been formed on the web. The transfer-assist blade is run over a surface of toner image fixed at a first printer. Thus, some of the toner adheres to the transfer-assist blade and accumulates on the blade surface, thus causing degradation of images on the first surface.
Generally, when an image is printed on a second surface of a web, the pressure load by the transfer-assist blade is needed to be higher to improve adhesion. Because the web may be deformed by receiving heat during the fixing process for the first surface of the first printer, the transfer-assist blade is increasingly soiled.
As other transfer devices, a roller transfer system and a belt transfer system are well known, their mechanisms having been adopted by cut sheet printers in a slow-speed or a middle-speed range. For example, Japanese Unexamined Patent Application Publication No. S48-51640 describes that a transfer driven-roller is pressed to a web by a swing lever. The roller transfer system is able to decrease a gap between irregularities of a web surface and a photoconductor as described above, relative to noncontact corotron-transfer system because the transfer roller is contacted to a web at a moderate pressure. The roller transfer system supplies electric charges to a transfer roller in order to transfer a toner image developed on a photoconductor to a surface of a web by electrostatic force. As ways of supplying electric charge to a transfer roller, it is known to apply current to the cored bar of a transfer roller, and also known to apply electric charge to the surface of transfer roller having a dielectric layer surface.
The system of applying current to the cored bar of a transfer roller is adopted by cut sheet printers in a range of low or middle speed. A surface charge for transfer is increased as the printing speed increasing. Thus, the nip width between a transfer roller and a web needs to be extended. To extend a nip width, there are some methods of extending roller diameter, increasing thickness of the elastic member on a roller, or decreasing the hardness of the elastic member, etc. However, extending the roller diameter or increasing the thickness of elastic member needs more space, and decreasing the hardness of the elastic member causes an extension of a roller deforming and decreasing the life of the roller.
The system of applying electric charge to the surface of transfer roller can prevent extension of the transfer roller diameter or decreasing the life of the roller, because the nip width doesn't increase according to print speed, and the dielectric layer on the surface also serves as a protection layer. Regarding this system, for example, Japanese Unexamined Patent Application Publication No. S52-42125 describes that electric charges are supplied onto the surface of a transfer-roller (or belt). However, the system of applying electric charge to the surface of the transfer roller needs a charger for supplying electric charge and an eliminator for eliminating electric charge on the surface of a transfer roller. Thus, costs increase.
In a case of applying a transfer roller to a continuous sheet printer, the following factors need to be considered. A transfer roller is generally driven by a web. In case of using cut sheets, the transfer roller can contact a photoconductor or an intermediate transfer belt before printing. Because cut sheets as a web are separated from each other, and printing can be completed to the fixing process, it is unusual that a web stops having toner thereon in an unfixed-state inside the apparatus.
In the case of continuous sheet, printing may also stop during a print job. In that case, unfixed-state toner images may exist. A web is normally fed back enough pages to ensure an entrance length for the next printing. During the printing start-up process, the web is accelerated to a prescribed speed. Thus, in the case of continuous sheets, a contacting/freeing mechanism for a transfer roller is needed. Because the transfer roller needs to release from the web during the printing restart process to avoid image defects of the unfixed-state toner images. An operation of the contacting/freeing mechanism should be performed during the period of non-printing area from an end of the print-stop page to a top of the print-restart page.
In cases like this, when the system of applying current to the cored bar of a transfer roller as above is used, transfer performance will be unaffected because of current flowing constantly from inside the roller, if a rotation speed of the transfer roller lags behind a feed speed of the web. However, in the case of the system applying electric charge to the surface of transfer roller as above, in the print starting-up process, the electric charges are not applied on the surface between from the applying point of electric charge to the transfer point because the transfer roller is driven. Thus, in case that the transfer roller is driven, there is a problem that transferring cannot be performed when a print process starts up, and then, the following capability of the transfer roller is no good in a range of high-speed.
As described above, the continuous sheet printer is also expected to adopt to a wide variety of webs to respond to various purposes. The widths of the web are decided as usage. For example, if a photoconductor of the long length (from 21 to 22 inch) is used, the widths of the web range with 12 inch, 16 inch, 22 inch, etc. The widths of a web range from 12 to 16 inch in continuous form printing, and the widths of a web range with 17 inch being cut by 2 paper of A4 size, or with 22 inch being cut by 3 paper of B5 size in manual or document printing. Thus, when printing by using a web of narrow width, the part of the photoconductor without a sheet passing contacts the transfer roller directly. In this regard, for example, Japanese Unexamined Patent Application Publication No. S52-42125 describes the surface-speed of the transfer member is changed by using a switch. In a case that the transfer roller is driven, there is no problem that both the transfer roller and the photoconductor are the same in speed. However, if the transfer roller and the photoconductor are both driven, a short life span of the transfer roller and the photoconductor may be caused by the mechanical friction between surfaces. In the case of the continuous sheet printer as described above, the movement of the transfer roller needs to be performed in a short time. Otherwise, the image degradation such as character blur may be caused by vibration, because the more speed and the more weight of the transfer roller, the higher the inertia will be.